1. Field of the Invention
This invention relates to an improved method and apparatus for the mixing and extrusion of thermoplastic and thermosetting materials, more particularly, this invention concerns an improved screw or worm type apparatus and method for the mixing, refining, separating and extruding variable viscosity natural and synthetic rubber and other elastic materials. Unless otherwise indicated the term extruder, as used herein, refers to and includes a screw or worm type apparatus used to obtain an extrusion of the desired cross-sectional configuration or for mixing, blending and processing of thermo-plastic and thermo-setting materials.
2. Description of Prior Art
To evaluate the level of prior art, the flow of the process material through the present mixing equipment is as follows:
THE TWO ROLL MILL with the material banded one roll generates a band of material of which only the outer part is worked, as the inner part adheres to the roll surface and thereby receives little or no working at all. Material arriving at the mill nip, as formed between the two rolls, has to "PUSH" its way into the mill bank and thereby relaxes any strain previously established in the material. When the material is in the bank, the shearing action is directed to the path of least resistance, the already worked material.
THE INTERNAL MIXER as used to assemble compounds, has two in running rotors with configurations designed to shear the baled polymer so as to develop surface area, to incorporate the additives uniformly, and to blend the batch to a uniform material. This multiple purpose design actually develops turbulent flow which directs the imposed shear to the path of least resistance, incorporates the additives with little relation to the designed amount, and the mixing at the center of the chamber exceeds that at the extremes. Post mixer blending at best produces a blend of over and under mixed material. Loose black dropped with the batch is the rule rather than the exception. Plateau type chemicals are required to produce usable material.
THE HELICAL GROOVE EXTRUDER imparts shear to material contacting the rotor grooves and barrel bore. This action moves the material at the outer part of the groove to flow from the leading edge to the trailing edge of the groove. Arriving at the training edge it must "PUSH" its way down and back under to arrive at the leading to complete the cycle. Also the imposed shear is directed to already worked material tending to create a cold core surrounded with warmed material, an action which generates non-uniform mixing.
THE TRANSFERMIX EXTRUDER which transfers the material from the rotor to the barrel and then back to the rotor with barrel grooves extending the length of the exchange. The first half receives the material from the rotor and the second half transfers the same material back to the rotor. The resulting action is one that winds in and winds out the same material, which tends to rework the same material. The net result is mixing inferior to mixing where newly aligned material is subjected to the barrel to rotor transfer.
THE BARRIER TYPE EXTRUDER has a fixed clearance between barrier tops and the barrel bore. As different materials and different viscosities may require different clearances, the effect of wear and the size of material to be rejected, etc, limits the use of the Barrier Screw to the thermoplastic industry.
Recognizing that the present mixing equipment can be improved requires new and improved mixing equipment that does not exist. Although laboratory equipment out performed factory mix, the expense of changing factory mixing equipment invited a status quo situation.